Tonnage is area, thickness and density
Area times thickness gives volume; volume times the stone\'s density gives weight; dividing by 2,000 converts pounds to US tons. Keep every measurement in one system before you convert. Standard riprap runs about 100 lb per cubic foot (roughly 1.4–1.5 tons per cubic yard), so a cubic yard of stone weighs a bit over a tonne.
Density by stone type
| Material | lb / cubic yard | Tons / cubic yard |
|---|---|---|
| Standard riprap (6–12") | ~4,050 | ~1.8 |
| Crushed stone | ~4,200 | ~1.9 |
| Granite | ~4,380 | ~2.0 |
| Limestone | ~4,545 | ~2.0 |
| Concrete rubble | ~6,750 | ~3.1 |
Denser rock means fewer cubic yards for the same tonnage — but sizing is set by the water velocity, not the weight, so pick the stone size first and let density fall out of it.
A worked example
- Area. A 30 ft by 20 ft bank = 600 sq ft.
- Thickness. 18 in (1.5 ft), which is 1.5 × a 12 in median stone.
- Volume. 600 × 1.5 = 900 cubic feet, about 33 cubic yards.
- Weight. 33 yd³ × ~1.5 tons/yd³ ≈ 50 tons. Order 5–10% extra for settling and the filter interface.
What people get wrong on quantity
- Skimping on thickness. Under 1.5 × the stone size, the layer unravels. Always meet the minimum 12 in.
- Forgetting the filter. Riprap needs geotextile or a gravel filter beneath it, or the soil pipes out through the gaps. That\'s a separate quantity to order.
- Mixing units. Feet with metres, or cubic yards with tons, is the usual source of a wildly wrong order. Convert only at the end.
- Ignoring frost and ice. In cold regions add 6–12 in of thickness, which raises the tonnage accordingly.
Common questions
How do I calculate tons of riprap?
Multiply the area by the layer thickness to get volume, then multiply by the stone density and divide by 2,000 for US tons. For 600 sq ft at 1.5 ft thick that is 900 cubic feet; at about 100 lb per cubic foot that is 90,000 lb, or 45 tons.
How thick should a riprap layer be?
At least 1.5 times the median stone size, and never less than 12 inches (about 300 mm). A layer thinner than that lets individual stones sit proud and wash out one by one. In freezing climates add another 6 to 12 inches.
How many tons of riprap are in a cubic yard?
About 1.4 to 1.5 tons per cubic yard for typical rock, and up to 2 tons for dense granite or limestone. One cubic yard is 27 cubic feet, so at roughly 100 lb per cubic foot that is around 2,700 lb, or 1.35 tons.
